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Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across. [45] In terms of numbers of individuals, Prochlorococcus is possibly the most plentiful genus on Earth: a single millilitre of surface seawater can contain 100,000 cells of this genus or more.
The colonial marine cyanobacterium Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. [52] Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen. [ 53 ]
Nitrogen enters the ocean through precipitation, runoff, or as N 2 from the atmosphere. Nitrogen cannot be utilized by phytoplankton as N 2 so it must undergo nitrogen fixation which is performed predominantly by cyanobacteria. [82] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 ...
Cyanobacteria usually obtain a fixed carbon (carbohydrate) by photosynthesis. The lack of water-splitting in photosystem II prevents heterocysts from performing photosynthesis, so the vegetative cells provide them with carbohydrates, which is thought to be sucrose. The fixed carbon and nitrogen sources are exchanged through channels between the ...
As a result, marine N 2-fixing microorganisms other than Trichodesimum were found by sequencing PCR-amplified fragments of the gene nitrogenase (nifH) .Nitrogenase is the enzyme that catalyzes nitrogen fixation, and studies have shown that nifH is widely distributed throughout the different parts of the ocean.
Nitrogen fixation in Trichodesmium is unique among diazotrophs because the process occurs concurrently with oxygen production (via photosynthesis [2]). In other cyanobacteria , N 2 and CO 2 reduction are separated either in space (using heterocysts to protect the sensitive nitrogenase enzyme from oxygen) or time.
The principal marine primary producers are cyanobacteria, algae and marine plants. The oxygen released as a by-product of photosynthesis is needed by nearly all living things to carry out cellular respiration. In addition, primary producers are influential in the global carbon and water cycles. They stabilize coastal areas and can provide ...
Relationship of phosphate to nitrate uptake for photosynthesis in various regions of the ocean. Note that nitrate is more often limiting than phosphate The Redfield ratio or Redfield stoichiometry is the consistent atomic ratio of carbon , nitrogen and phosphorus found in marine phytoplankton and throughout the deep oceans.